That is a whole lot of food. To be quite honest, there's a couple things I want to mention. 1. Being that the food has some rather questionable ingredients. There are several things that you don't want to really see in dog food and it has several (corn, corn gluten meal). 2. It has grains and chicken, both of which some mastiffs have issues with. Yours might not but keep an eye out for things like perpetually loose stool, and extreme gas. 3. You do not need a large breed puppy food, a large breed food or a puppy food. A quality adult or all life stages food is more likely to meet the criteria that a food needs to be allow for slow quality growth in a mastiff pup. Also the better the food, usually the less you have to feed. I mean I'm feeding half what their recommendation is (8.5 cups) on my food for a puppy in the same weight range. My food also has a little more calories so she definitely isn't starving by any means. If you're truly concerned, (whether it's for your dog or your wallet since it would seem your going through a lot of food haha), I suggest switching foods. Keep in mind though you will need to transition between the old and the new.
and I do to less mess.